1. Field of the Invention
The present invention relates to an optical fiber and, more particularly, to a broad band dispersion-controlled optical fiber.
2. Description of the Related Art
As one skilled in the art can readily appreciate, an optical fiber consists of a core and a cladding, wherein the refractive index of the core is higher than that of the cladding. Common known methods for manufacturing the base material of an optical fiber includes the Modified-Chemical-Vapor Deposition (MCVD) method, Vapor-phase Axial Deposition (VAD) method, Outside Vapor-phase Deposition(OVD) method, Plasma-Chemical-Vapor Deposition(PCVD) method and the like.
For achieving ultra-high speed and high capacity communication, dispersion-controlled optical fibers (for example, dispersion-shifted fiber (DSF), non-zero DSF (NZDSF), dispersion-compensated fiber (DSF)) have been deployed which are superior to the existing single-mode optical fiber in terms of transmission capability. As such, the demand for the dispersion-controlled fibers has been increasing. If a region with a depressed refractive index is interposed between the core and cladding to form an optical fiber, it is possible to effectively control the dispersion characteristics of the optical fiber. An example of such an optical fiber is disclosed in U.S. Pat. No. 4,715,679 to Venkata A. Bhagavatula, entitled “Low Dispersion, Low-loss Single-mode Optical Waveguide.”
However, the dispersion-controlled optical fiber of this type has drawbacks in that its bending loss tends to be high as it has a region with a highly depressed refractive index in its cladding. In addition, a non-linear effect occurs due to its small effective cross-sectional area as it has a small mode-field diameter (MFD) when compared to common single-mode optical fibers. Furthermore, it is inappropriate for broad-band transmission, and the loss and dispersion characteristics are poor in higher and lower wavelength ranges.
A dispersion-controlled optical fiber has a very small core diameter and high refractive index when compared to a single-mode optical fiber. As such, if the dimension of its base material forms a large aperture, a problem will arise as relatively large stresses are applied to the core part at the time of drawing it. Namely, the distribution of wavelengths will be changed. This means that it is difficult for various optical characteristics to have constant values in accordance with drawing temperatures. Also, it is not easy to manufacture a dispersion-controlled optical fiber if it has relatively sensitive characteristics when compared to a common single-mode optical fiber.
In addition, the existing dispersion-controlled optical fibers are adapted to be used in the wavelength range of about 1530˜1565 nm by setting the zero dispersion wavelength around 1530 nm, wherein the optical fibers have a dispersion characteristic of not more than 5 ps/nm·km at 1550 nm and their diameters range between 8˜9 μm, thus being problematic in that they are inappropriate for communication exceeding the 10 Gbps level.
As explained above, dispersion-controlled optical fibers in the prior art have the following problems:                a) the existing dispersion-controlled optical fibers, such as a dispersion-compensated fiber, dispersion-shifted fiber, non-zero dispersion-shifted fiber, use a small wavelength window as the zero dispersion is positioned adjacent to 1530 nm, thus not suitable for use in high capacity transmission;        b) an optical fiber of low dispersion has the problem of exhibiting a small dispersion characteristic, i.e., a non-linear effect (four-wave mixing (FWM), and a cross-phase modulation (XPM)) is generated at the time of super-high speed transmission;        c) a common single-mode optical fiber has the problem of exhibiting an overly large dispersion (≧17 ps/nm·km) characteristic in the EDF window, thus a non-linear effect (self phase modulation (SPM)) is produced; and,        d) if an optical fiber has a high core-refractive index and a small core diameter in order to control the dispersion characteristic, a problem may arise in that it may be greatly influenced by a non-linear effect as it has a small mode-field diameter (effective cross-sectional area at 1550 nm<50 μm2). In addition, there is a problem in that the aforementioned non-linear effect is further amplified if the dispersion value is either too large or too small (XPM, SPM and FWM have a trade-off relationship), thereby deteriorating transmission characteristics.        